The proposed research will utilize behavioral and molecular genetic techniques to examine the etiology of DSM-IV Attention Deficit/Hyperactivity Disorder (ADHD) in a sample of 100 8-13 year old twins selected because at least one twin meets the criteria for ADHD. Univariate behavioral genetic models will be utilized to estimate the genetic and environmental components of the etiology of the subtypes of ADHD. In addition, bivariate behavior genetic analyses will be utilized to directly examine the influence of common genes on the three proposed subtypes and the etiology of any observed psychiatric comorbidity with ADHD. DNA samples will be obtained from all DZ twins and their families. Association analyses such as the Transmission Disequilibrium Test will be utilized to test whether any component of the ADHD phenotype is significantly associated with any of several theoretically plausible candidate genes. In addition, a bivariate interval mapping linkage analysis will be utilized in a sample of individuals already typed for the relevant markers to test whether a previously mapped quantitative trait locus for reading disability on chromosome 6 is also etiologically related to ADHD.